Production of elemental films using a boron-containing reducing agent

ABSTRACT

The present invention relates generally to depositing elemental thin films. In particular, the invention concerns a method of growing elemental metal thin films by Atomic Layer Deposition (ALD) using a boron compound as a reducing agent. In a preferred embodiment the method comprises introducing vapor phase pulses of at least one metal source compound and at least one boron source compound into a reaction space that contains a substrate on which the metal thin film is to be deposited. Preferably the boron compound is capable of reducing the adsorbed portion of the metal source compound into its elemental electrical state.

REFERENCE TO RELATED APPLICATIONS

The present application claims the priority benefit under 35 U.S.C. §120as a continuation of U.S. application Ser. No. 10/990,424, filed Nov,16, 2004, now U.S. Pat. No. 7,144,809, issued Dec. 5, 2006, which isitself a continuation of U.S. application Ser. No. 10/210,715, filedJul. 30, 2002, now U.S. Pat. No. 6,821,889, issued Nov. 23, 2004, whichis in turn a continuation of U.S. Application Ser. No. 09/687,205, filedOct. 13, 2000, now U.S. Pat. No. 6,475,276, issued Nov. 5, 2002 andunder 35 U.S.C. §119(e) to U.S. Provisional Application Nos. 60/176,948,filed Jan. 18, 2000, and 60/159,799 filed Oct. 15, 1999 and under 35U.S.C. §119(a) to Finnish Application Nos. FI 19992233, filed Oct. 15,1999, FI19992234 filed Oct. 15, 1999, FI19992235, filed Oct. 15, 1999and FI20000564, filed Mar. 10, 2000. Each of the priority applicationsis hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method of depositing thinfilms. In particular, the invention concerns a method of growingelemental metal thin films by Atomic Layer Deposition (ALD) using aboron compound as a reducing agent.

2. Description of the Related Art

The integration level of components in integrated circuits isincreasing, producing a need for smaller components and interconnects.Design rules are dictating a feature size less than or equal to 0.2 μm.This makes complete film coverage on deep vias difficult to obtain.

Integrated circuits contain interconnects that are conventionally madeof aluminum. Today, copper is replacing aluminum because it has lowerelectrical resistance and better electromigration resistance thanaluminum.

Chemical Vapor Deposition (CVD) has been commonly used to produce metalfilms. In CVD, the source materials are typically fed into a reactionspace together, where they react when brought into contact with a hotsubstrate. Thus, the growth rate of the metal film depends in part uponthe concentration of the different source materials in the reactionspace. Additionally, the temperature of the substrate affects the rateof deposition. In thermal CVD a single source chemical can be thermallydecomposed near the substrate.

Atomic Layer Deposition (ALD) is an advanced alternative to CVD. The ALDmethod is based on sequential self-saturating surface reactions and hasbeen described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811.Source chemicals are pulsed into the reaction chamber in an inertcarrier gas. The pulses of source chemical can be separated from eachother by a purging flow of inert gas. The separation of the sourcechemicals and the proper choice of source chemicals prevents gas-phasereactions between gaseous reactants and enables self-saturating surfacereactions. This allows for film growth without strict temperaturecontrol of the substrate or precise dosage control of the reactants.Surplus reactants and byproducts are removed from the chamber, such asby a purging flow of inert gas, before the next reactive chemical pulseis introduced. Undesired gaseous molecules are effectively removed fromthe reaction chamber by keeping the gas flow speeds high. The purginggas pushes the extra molecules towards the vacuum pump that is used tomaintain a suitable pressure in the reaction chamber. Thus, ALD providesfor rapid, uniform, controlled film growth.

While ALD has been used to produce both elemental and compound thinfilms, there are a number of drawbacks to the methods that have beenused. Thus, a need exists for improvements in the production of metalthin films.

SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a method of growingelemental thin films on a substrate by an atomic layer deposition (ALD)type process. The method comprises introducing vapor phase pulses of atleast one elemental source compound and at least one boron sourcecompound into a reaction space that contains a substrate on which thethin film is to be deposited.

The vapor phase pulses are alternately introduced in a cycle. Each cyclecomprises introducing an elemental source compound into a reaction spacecontaining a substrate, removing any gaseous compounds from the reactionspace, introducing a boron source compound into the reaction space, andremoving any gaseous compounds from the reaction space. The elementalsource compound preferably reacts with the surface of the substrateproducing a surface bound elemental compound. Preferably the boronsource compound is capable of reducing the surface bound elementalcompound into elemental form.

In the preferred embodiment a metal source compound is used and anelemental metal thin film is grown on the substrate. The metal sourcecompound and boron source compound are fed into the reaction chamberwith the aid of an inert carrier gas. An inert gas may also be used topurge the reaction space after each pulse of metal source compound andboron compound.

In one embodiment the boron compound contains at least one carbon atomand the elemental source compound comprises at least one metal selectedfrom the group consisting of Cu, Ag, Au, Pd, Rh and/or Ir. In anotherembodiment the boron compound contains no carbon atoms and the elementalsource compound comprises at least one metal selected from the groupconsisting of Cu, Ag, Au, Pd, Rh, Ir, Ti, Zr, Hf, V, Nb, Ta, Cr, Moand/or W.

In accordance with one aspect of the invention, an electron conductor isproduced by an ALD type process wherein a boron compound is used toreduce a surface bound elemental compound to its elemental state. Inaccordance with another aspect of the invention, an interconnect isproduced in an integrated circuit by depositing a metal thin film by anALD type process wherein a boron compound is used to reduce a surfacebound metal compound to its elemental state. In yet another embodiment ametal seed layer is grown on a substrate by growing an elemental metalthin film on a substrate by an ALD type process wherein a boron compoundis used to reduce a surface bound metal compound to its elemental state.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 presents a block diagram of a pulsing sequence for producingelemental films according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An “ALD type process” is a process in which deposition of material ontoa surface is produced by sequential and alternating self-saturatingsurface reactions. The principles of ALD are disclosed in U.S. Pat. Nos.4,058,430 and 5,711,811, the disclosures of which are incorporatedherein by reference.

“Reaction space” is used to designate the portion(s) a reactor orreaction chamber in which the conditions can be adjusted so thatdeposition by ALD is possible.

“Thin film” is used to designate a film that is grown from elements orcompounds that are transported as separate ions, atoms or molecules fromthe source to the substrate. The thin film may be, for example, anelemental metal thin film. However, one skilled in the art willrecognize that the principles and advantages disclosed herein areapplicable to elemental thin films other than metal. The inventors havefound the processes disclosed herein particularly advantageous fordepositing metal layers. The thickness of the film depends on theapplication and may vary in a wide range. For example, an elementalmetal thin film may range from one atomic layer to 1,000 nm inthickness.

“Elemental thin film” means a thin film with constituents having anoxidation state of zero. The elemental thin films disclosed hereincomprise elemental metals and metal alloys.

An “elemental source compound” or material is a compound that comprisesat least one molecule of the element that is to comprise the desiredelemental thin film. In the illustrated embodiments, the elementalsource compound is a vapor-phase metal source compound.

A “boron reducing compound” is any boron-containing compound that iscapable of reducing an elemental compound to its elemental state.

An “organic boron compound” is a boron-containing compound thatcomprises at least one carbon atom.

A “surface bound elemental complex” is an adsorbed species upon thesubstrate that contains the element that is to form the desired thinfilm, in addition to terminating ligands or tails. The surface boundelemental complex (e.g., chemisorbed metal complex) can be the elementalsource compound or a portion thereof.

An “adsorbed metal complex” or “chemisorbed metal complex,” as usedherein, denote a compound including both a metal atom and terminatingligand(s) such as halide or organic tails.

According to the present invention, a chemical gaseous depositionprocess is used to produce metal thin films. Preferably, this process isan ALD type process.

According to a preferred embodiment of the present invention, elementalmetal thin films are prepared by an ALD type process. Surface bound orchemisorbed metal complexes are reduced by boron compounds intoelemental form. Thus, in one embodiment of the invention, a substrate isfirst placed in a reaction chamber where it is subjected to alternatelyrepeated surface reactions of at least two vapor-phase reactants.Preferably the substrate is maintained at an elevated temperature. Theboron reducing compounds preferably are not incorporated appreciablyinto the resulting thin film.

In the ALD type process of the preferred embodiment, the conditions inthe reaction space are adjusted so that gas-phase reactions are avoided.Reactions are limited to adsorption on the substrate in one phase, andreactions that occur between complex adsorbed on the surface of thesubstrate and a gaseous reactant in another phase. Thus, the moleculesof the boron reducing compound react with the surface bound elementalcomplex on the surface of the substrate.

In the preferred embodiment, vapor-phase pulses of metal source materialand the boron reducing agent are alternately and sequentially fed intothe reaction space where they contact the surface of the substrate.Initially, the “surface” of the substrate comprises the actual substratematerial. Alternately, the substrate may be pretreated in advance. Forexample, the substrate may be contacted with a chemical that modifiesthe surface properties of the substrate. During the growing of the metalthin films, the previous thin film layer forms the surface for anysubsequent thin film layer.

The metal source material and boron reducing agent are preferably fedinto the reaction chamber in pulses with the aid of an inert carriergas. In one embodiment, each pulse is followed by an inert gas pulsethat purges any unreacted residues or byproducts from the reactionchamber. This allows for the use of highly reactive chemicals and thuslow deposition temperatures. The inert gas used in the purging pulse ispreferably the same gas used as the carrier gas. The inert gas maycomprise an inactive gas, such as nitrogen or a noble gas, such asargon.

According to one embodiment, a mild reducing agent is added to the inertgas purge in order to prevent the possible reoxidation of the substratesurface. The reducing agent is preferably used in a concentration of0.1% to 10%, more preferably 0.5% to 5% and even more preferably 0.5% to1% by volume of the inert gas. The agent is selected so that it will nothave a detrimental effect on the substrate surface. In one embodiment,hydrogen is used as the mild reducing agent.

Thus, one sequence or “cycle” in the process of depositing metal thinfilms preferably consists of:

1. Feeding a vapor phase pulse of an elemental source chemical into thereaction space with the help of an inert carrier gas;

2. Purging the reaction space with an inert gas;

3. Feeding a vapor-phase pulse of a boron source chemical into thereaction space with the help of an inert carrier gas; and

4. Purging the reaction space with an inert gas.

The above-described cycle can be repeated to produce metal films of thedesired thickness.

The deposition can be carried out at atmospheric pressure. Preferably,the deposition is carried out at a reduced pressure of 0.01 mbar to 20mbar, more preferably 0.1 mbar to 5 mbar. The substrate temperature ispreferably low enough to keep the bonds between metal atoms intact andto prevent thermal decomposition of the gaseous reactants. On the otherhand, the substrate temperature is preferably high enough to keep thesource materials in the gaseous phase. Condensation of the gaseousreactants is preferably avoided. Further, the temperature is preferablysufficiently high to provide the required activation energy for thesurface reaction. The preferred temperature depends upon the specificreactants and pressure. However the temperature of the substrate ispreferably between 100° C. and 700° C. and more preferably between 250°C. and 500° C.

The source temperature is preferably set below the substratetemperature. If the partial pressure of the source chemical vaporexceeds the condensation limit at the substrate temperature,condensation may occur and the controlled layer-by-layer growth of thefilm may be lost.

Under the preferred conditions described above, at least a portion ofthe metal source reactant will bind to the substrate surface throughchemisorption. Maximum coverage is obtained when a single layer ofsurface bound metal complex is formed. At this point there are no morebinding sites available for the metal source compound and adsorptionceases. Thus, the amount of reactant bound to the surface of thesubstrate will be limited by self-saturation and the maximum increase inthin film thickness is one atomic layer per pulsing sequence. Dependingon the size of the surface bound or chemisorbed metal complex, theincrease in thin film thickness can be less than one atomic layer perpulsing sequence on average. The pulsing sequence is repeated to producea thin film of the desired thickness.

The amount of time available for the self-saturating reactions islimited mostly by economic factors, such as a required throughput ofproduct. Very thin films may be made by relatively few pulsing cycles.In some cases this will allow for an increase in the source chemicalpulse time and thus allow for the use of source compounds with a lowervapor pressure.

The substrate can be any material known in the art. Examples includesilicon, silica, coated silicon, copper metal and nitrides, such asmetal nitrides. A typical substrate is a silicon wafer coated withnitrides.

The present method provides an excellent way to grow conformal layers ingeometrically challenging applications. For example, elemental metalfilms may be grown on vias or trenches. According to one embodiment ofthe present invention, a metal thin film is grown on TiN or TaN oranother suitable nitride that forms a nucleating layer to which metalscan attach. For example, an elemental metal thin film may be grown overa metal nitride diffusion barrier in a dual damascene structure.Thereafter a film of desired form (e.g. copper film) can be grown by anelectrolytic method.

The elemental source materials most typically used to grow elementalthin films are preferably volatile or gaseous compounds of thetransition metals of groups 3, 4, 5, 6, 7, 8, 9, 10, 11 and/or 12(according to the IUPAC system) in the periodic table of the elements.In particular, the elemental metal thin films consist essentially of oneor more of W, Cu, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Pd, Pt, Rh, Ir, Ag andAu.

Since the properties of each elemental compound are different, theirsuitability for use in the method of the present invention must beindividually assessed in light of the other reactants that will be used.The properties of the elemental compounds are known to the skilledartisan and may be found, for example, in N. N. Greenwood and A.Earnshaw, Chemistry of the Elements, 1^(st) Edition, Pergamon Press,1986. Metal compounds that contain Cu, Ag, Au, Pd, Pt, Rh, Ir, Ti, Zr,Hf, V, Nb, Ta, Cr, Mo, and/or W can be alternated in ALD type reactionswith boron compounds that do not contain carbon. Metal compounds thatcontain Cu, Ag, Au, Pd, Pt, Rh and/or Ir are preferably used in ALD typereactions with organic boron compounds.

Preferably the metal source compound will be chosen so that therequirements of sufficient vapor pressure, sufficient thermal stabilityat the substrate temperature and sufficient reactivity are all met.Sufficient vapor pressure means that there must be enough sourcecompound molecules in the gas phase near the substrate surface to enableself-saturating reactions at the surface. Sufficient thermal stabilitymeans that the source compound itself must not undergo thermaldecomposition to form unwanted impurities in the thin film orcondensation that disturbs thin film growth.

Several other factors that may be considered in selecting the metal (orother elemental) source compound include the availability of thecompound in a highly pure form and the ease of handling the material.

In addition, for the formation of elemental metal thin films, the vaporpressure of the elemental metal must be low enough under the conditionsin the reactor space that the rate of evaporation of the thin film islower than the rate of formation from the source compound.

Preferred metal source materials may be found, for example, among metalhalides, preferably fluorides, chlorides, bromides or iodides. They mayalso be found among metal organic compounds, such as alkylaminos,cyclopentadienyls, dithiocarbamates or betadiketonates of the desiredmetal.

In one embodiment of the present invention, tungsten metal thin filmsare grown. In this embodiment, one or more of the following compoundsare preferably used:

-   -   halides, such as WF₆, WCl₆, WCl₄, and WBr₅;    -   carbonyls, such as W(CO)₆ and tricarbonyl(mesitylene)tungsten;    -   cyclopentadienyls, such as bis(cyclopentadienyl)tungsten        dihydride, bis(cyclopentadienyl)tungsten dichloride and        bis(cyclopentadienyl)ditungsten hexacarbonyl.

A preferred tungsten source material is WF₆; A boron compound that doesnot contain carbon is preferably used as a reducing agent.

According to another embodiment of the present invention, copper metalthin films are grown. For the metal source material, one or more of thefollowing compounds are preferably used:

-   -   halides, such as CuCl, CuBr and CuI;    -   compounds where copper is coordinated to oxygen, such as        bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper, copper(II) 2        ethylhexanoate, bis(2,4-pentanedionato)copper and their        derivatives such as        bis(1,1,1-trifluoro-2,4-pentanedionato)copper and        bis(ethylacetoacetonato)copper;    -   compounds where copper is coordinated to sulfur, such as        copper(1)-butanethiolate and copper dialkyldithiocarbamates.

A preferred copper source material is CuCl. The suggested reactionequations for the deposition of copper from CuCl and TEB are presentedbelow (R1-R3).2CuCl(ads)+(CH₃CH₂)₃B→2Cu(ads)+(CH₃CH₂)₂BCl(g)+CH₃CH₂Cl(g)  (R1)2CuCl(ads)+(CH₃CH₂)₂BCl(g)→2Cu(ads)+(CH₃CH₂)BCl₂ (g)+CH₃CH₂Cl(g)  (R2)2CuCl(ads)+(CH₃CH₂)BCl₂(g)→2Cu(ads)+BCl₃(g)+CH₃CH₂Cl(g)tm (R3)

In addition to thin films comprised of a single metal, the presentinvention contemplates thin films comprising two or more metals. In thisway it is possible to achieve a thin film that has the beneficialproperties of more than one metal, such as the good conductivity of onemetal and the good corrosion resistance of another. For example, thinfilms that contain both titanium and tungsten provide a barrier layerfor preventing the interdiffusion of copper into silicon anddielectrics.

When the elemental source material reacts with the substrate, a covalentbond is formed with the surface bonding groups. The surface boundelemental complex (e.g., chemisorbed metal complex) is surfaceterminated, such as with halogen or hydrocarbon tails that are notfurther reactive with the elemental source material. According to themethod of the present invention, the surface bound elemental complex isreduced in a reaction with a gaseous boron compound.

The boron source compound is chosen using the same criteria as for themetal source compound, as described above. In general, the boron sourcecompound may be any volatile, sufficiently thermally stable boroncompound that is capable of reducing the surface bound elementalcomplex. The reducing strengths of boron compounds vary. Thus, for thedeposition of metal thin films, it is preferable to use a boron compoundthat is able to reduce the chemisorbed metal complex to its elementalstate. Organic boron compounds are preferably used to reduce metalcomplexes containing Cu, Ag, Au, Pd, Pt, Rh and/or Ir, while boroncompounds that do not contain carbon atoms are preferably used to reducemetal complexes containing Cu, Ag, Au, Pd, Pt, Rh, Ir, Ti, Zr, Hf, V,Nb, Ta, Cr, Mo and/or W.

The reaction of different surface bound elemental complexes with thesame reducing agent leads to different reaction products. In thepreferred embodiment of the present invention, the elemental sourcecompound and boron compound are selected so that the byproductsresulting from the reaction of the surface bound elemental complex andthe boron compound are gaseous. The byproducts are preferably removedfrom the reaction space with inert gas during the purging pulse. Inaddition, the byproducts preferably do not decompose catalytically orthermally to condensable species. In this way the incorporation ofbyproducts, such as boride, into the thin films as impurities isavoided.

Selection of the elemental source compound and the boron compoundaccording to the above criteria allows for the progressive growth ofthin films by successive reaction sequences without a decrease in thegrowth rate caused by contamination of reactive sites on the substratesurface. Preferably the growth rate decreases by a maximum of 0.1% percycle, more preferably by less than 0.01% and even more preferably byless than 0.001% per cycle.

The selection of elemental source compounds and boron reducing compoundscan be facilitated with computer programs having a sufficientlyextensive thermodynamics database. This enables one skilled in the artto check the reaction equilibrium and predict which reactants havethermodynamically favorable reactions. An example of this type ofprogram is HSC Chemistry, Version 3.02, available from OutokumpuResearch Oy of Pori, Finland.

The availability of a vast number of boron compounds makes it possibleto choose one with the desired reducing strength while avoidingundesirable byproducts such as boride. In addition, it is possible touse more than one boron compound in the production of a single thinfilm.

Preferably, one or more of the following boron compounds is used:

Boranes according to formula I or formula II.B_(n)H_(n+x),  (I)

Wherein n is an integer from 1 to 10, preferably from 2 to 6, and x isan even integer, preferably 4, 6 or 8.B_(n)H_(m)  (II)

Wherein n is an integer from 1 to 10, preferably form 2 to 6, and m isan integer different than n, from 1 to 10, preferably from 2 to 6.

Of the above boranes according to formula I, examples includenido-boranes (B_(n)H_(n+4)), arachno-boranes (B_(n)H_(n+6)) andhyph-boranes (B_(n)H_(n+8)). Of the boranes according to formula II,examples include conjuncto-boranes (B_(n)H_(m)). Also, borane complexessuch as (CH₃CH₂)₃N—BH₃ can be used.

Borane halides, particularly fluorides, bromides and chlorides.

An example of a suitable compound is B₂H₅Br. Further examples compriseboron halides with a high boron/halide ratio, such as B₂F₄, B₂Cl₄ andB₂Br₄. It is also possible to use borane halide complexes.

Halogenoboranes according to formula III.B_(n)X_(n)  (III)

Wherein X is Cl or Br and n is 4 or an integer from 8 to 12 when X isCl, or n is an integer from 7 to 10 when X is Br.

Carboranes according to formula IV.C₂B_(n)H_(n+x)  (IV)

Wherein n is an integer from 1 to 10, preferably from 2 to 6, and x isan even integer, preferably 2, 4 or 6.

Examples of carboranes according to formula IV include closo-carboranes(C₂B_(n)H_(n+2)), nido-carboranes (C₂B_(n)H_(n+4)) andarachno-carboranes (C₂B_(n)H_(n+6)).

Amine-borane adducts according to formula V.R₃NBX₃  (V)

Wherein R is linear or branched C1 to C10, preferably C1 to C4 alkyl orH, and X is linear or branched C1 to C10, preferably C1 to C4 alkyl, Hor halogen.

Aminoboranes where one or more of the substituents on B is an aminogroup according to formula VI.R₂N  (VI)

Wherein R is linear or branched C1 to C10, preferably C1 to C4 alkyl orsubstituted or unsubstituted aryl group.

An example of a suitable aminoborane is (CH₃)₂NB(CH₃)₂.

Cyclic borazine (—BH—NH—)₃ and its volatile derivatives.

Alkyl borons or alkyl boranes, wherein the alkyl is typically linear orbranced C1 to C10 alkyl, preferably C2 to C4 alkyl.

In addition to the boron compounds described above, it is contemplatedthat silicon compounds may serve a similar function in the presentinvention.

In one embodiment of the present invention, elemental metal thin filmsare formed. In this case, the thin film is complete once the boroncompound reduces the metal to its elemental state.

EXAMPLES

The following examples illustrate the invention but do not limit thescope of the invention in any way.

Example 1

Copper chloride is pulsed into an ALD reaction chamber until heatedsubstrate surfaces are saturated with adsorbed CuCl molecules. Thesubstrate temperature is low enough to keep source compounds and bondswithin adsorbed complex intact, but high enough to prevent condensationof CuCl. The reaction chamber is then purged with inert nitrogen gasuntil the surplus CuCl has been removed. TEB is then pulsed into thereaction chamber until surface reactions are complete. Examples ofpossible reaction equations are presented in R2 to R4. Purging thereaction chamber of any surplus TEB and reaction byproducts with inertgas completes the reaction sequence. The reaction sequence is repeateduntil a metal film of the desired thickness is produced. The substratetemperature is maintained low enough that the copper-copper bond formedin the adsorption of CuCl to the previous thin film layer remainsintact.

A 50 mm by 50 mm piece of a silicon wafer and a 50 mm by 50 mm glasssubstrate were loaded into an ALD reactor. The substrates were heated to350° C. in a flowing nitrogen atmosphere (500 std. cm³/min) with apressure of about 10 mbar. Nitrogen gas was used as a carrier for thesource chemicals and as a purging gas. The carrier and pulsing gas mayinclude a mild reducing agent such as hydrogen gas to avoid thereoxidation of the copper surface. The pulsing cycle consisted of thefollowing steps:

CuCl vapor pulse for 0.3 seconds.

N₂ gas purge for 1.0 seconds.

(CH₃CH₂)₃B vapor pulse for 0.1 seconds.

N₂ gas purge for 1.0 seconds.

The pulsing cycle was repeated 1000 times. The resulting thin film had areddish metallic luster and was electrically conductive.

One skilled in the art will recognize that many variations are possibleusing the disclosed invention. The present invention is not limited toone particular embodiment, and the embodiments disclosed herein do notlimit the scope of the invention in any way.

1. A method of growing an elemental metal thin film on a substrate in areaction space by a vapor phase deposition process comprising multipledeposition cycles, each cycle comprising: pulsing a vapor phase metalsource compound into the reaction space such that no more than onemolecular layer of the metal source compound adsorbs on the substrateand does not thermally decompose; stopping the pulse of the metal sourcecompound; after stopping the pulse of the metal source compound, pulsinga vapor phase boron reducing compound into the reaction space in orderto reduce adsorbed metal source compound to elemental metal; feeding aninert gas pulse to the reaction chamber after each pulse of metal sourcecompound and boron reducing compound; and adding a mild reducing agentto the inert gas pulse.
 2. The method of claim 1, wherein the depositioncycle is repeated to form an elemental metal film of a desiredthickness.
 3. The method of claim 2, wherein the deposition cycle isrepeated until an elemental metal film with a thickness of from oneatomic layer to 1000 nm is formed.
 4. The method of claim 1, wherein themetal source compound and boron reducing compound are fed into thereaction chamber with the aid of an inert carrier gas.
 5. The method ofclaim 1, wherein the inert gas pulse comprises 0.5% to 1% of the mildreducing agent by volume.
 6. The method of claim 5, wherein the mildreducing agent is hydrogen.
 7. The method of claim 1, wherein the boronreducing compound contains no carbon atoms.
 8. The method of claim 7,wherein the boron reducing compound is selected from the groupconsisting of boranes of formula B_(n)H_(n+x), wherein n is an integerfrom 1 to 10 and x is an even integer.
 9. The method of claim 8, whereinthe boron reducing compound is selected from the group consisting ofnido-boranes of formula B_(n)H_(n+4), arachno-boranes of the formulaB_(n)H_(n+6), hypho-boranes of the formula B_(n)H_(n+8) andconjuncto-boranes B_(n)H_(m), wherein n is an integer from 1 to 10 and mis an integer from 1 to 10 that is different from n.
 10. The method ofclaim 9, wherein the boron compound is diborane.
 11. The method of claim1, wherein the metal source compound is a tungsten compound.
 12. Themethod of claim 11, wherein the tungsten compound is tungstenhexafluoride (WF₆).
 13. The method of claim 1, wherein the reactionspace temperature is between about 100° C. and about 700° C.
 14. Themethod of claim 1, wherein the reaction space temperature is betweenabout 250° C. and about 500° C.
 15. A method of growing an elementalmetal thin film on a substrate from vapor phase reactants in an atomiclayer deposition (ALD) process comprising: introducing a vapor phasemetal source compound into a reaction space containing the substratesuch that no more than one molecular layer of the metal source compoundadsorbs on the substrate; contacting the adsorbed metal compound on thesubstrate with a vapor phase boron source compound after introducing thevapor phase metal source compound such that no more than one atomiclayer of elemental metal is formed and such that no vapor phasereactions occur between the metal source compound and the boroncompound; feeding an inert gas pulse to the reaction chamber after eachpulse of metal source compound and boron compound; and adding a mildreducing agent to the inert gas pulse.
 16. The method of claim 15,wherein the deposition cycle is repeated until an elemental metal filmwith a thickness of from one atomic layer to 1000 nm is formed.
 17. Themethod of claim 15, wherein the metal source compound and boron compoundare fed into the reaction chamber with the aid of an inert carrier gas.18. The method of claim 15, wherein the mild reducing agent is hydrogen.19. The method of claim 15, wherein the reaction space temperature isbetween about 250° C. and about 500° C.
 20. The method of claim 15,wherein the boron compound is selected from the group consisting ofnido-boranes of formula B_(n)H_(n+4), arachno-boranes of the formulaB_(n)H_(n+6), hypho-boranes of the formula B_(n)H_(n+8) andconjuncto-boranes B_(n)H_(m), wherein n is an integer from 1 to 10 and mis an integer from 1 to 10 that is different from n.
 21. The method ofclaim 20, wherein the boron compound is diborane.
 22. The method ofclaim 15, wherein the metal source compound is a tungsten compound. 23.The method of claim 22, wherein the tungsten compound is tungstenhexafluoride (WF₆).